| 摘要 |
In transmission of facsimile signals, a known technique is that an image signal having a polarity with a minimum amplitude for one information state (white or black) and another amplitude for another information state (black or white), is converted before transmission into a 3-level signal, that is a signal of changing polarity. This saves transmission bandwidth. The method of conversion is that each change of the image signal causes a reversing pulse, and reversing pulses successively cause a change to one polarity, to the null state, to the other polarity, to the null state, etc. However, with this method, short black or white marks, which give an image signal of insufficient amplitude for reversing (Fig. 6a), are lost (Fig. 6b). To avoid this, before it is used to form the reversing pulses, the image signal is subjected in this method to amplitude splitting (Fig. 7a) and then compared with a threshold value to obtain the reversing pulses. In this way, image signals with an amplitude which would otherwise be insufficient are not lost (Fig. 7b). <IMAGE> |
